There are many methods and devices which use ultrasonic waves to measure the tensile load on a load-bearing-member (such as a fastener). Specifically, ultrasonic techniques can be used to provide a precise measure of the tensile load in a fastener (such as a bolt) during its installation or subsequently for purposes of inspection. These techniques are well documented in the prior art. In addition, techniques have been documented which use ultrasonic waves to detect flaws and for other nondestructive evaluation purposes in fasteners and other load-bearing members. The success of all of these techniques depends upon reliably detecting echo signals from within the parts under evaluation.
Several patents have described the use of a pulse-echo technique to measure the stress in a load-bearing member. U.S. Pat. No. 4,294,122 (issued to Couchman) discloses a fastener having an acoustic transducer built into its head or threaded end. The acoustic transducer is used to obtain pre-loading measurements and to provide improved quality control inspection of fasteners. A power-driven wrench incorporating a spring-loaded electrical connection to the transducer is electrically connected to a pulse-echo measuring system. The '122 patent also discloses a pulse-echo technique for measuring pre-load stress. The method includes measuring the time for two sets of echoes to travel the length of the fastener, one set before pre-load and the other set as torque is applied to the fastener. Then, by knowing the material constant M, the grip length .delta., the diameter D, an empirical parameter which corrects for stress distribution in fasteners .alpha., and the time difference in the travel time of the echoes .DELTA.T, the stress S can be measured to obtain an accurate measure of fastener pre-load by using the following formula: S=(M/(.delta.+.alpha.D)).times..DELTA.T.
Another patent disclosing the pulse-echo time measurement technique is U.S. Pat. No. 4,471,657 (issued to Voris et al.). The '657 patent discloses an apparatus and method for measuring the length of and stress in a tensile load member. The method includes measuring the time it takes two signals having the same frequency but a pre-determined phase difference to travel the length of a load-bearing member; detecting the longer of the travel times; compensating for the phase difference; and using an intelligent processing and control means to receive the time interval data and process the data to produce an accurate conversion to the change in fastener length or the stress applied to the load-bearing member. The apparatus includes an ultrasonic transducer permanently or temporarily in contact with the load-bearing member.
U.S. Pat. No. 4,602,511 (issued to Holt) teaches a method using the time-of-flight of both longitudinal and transverse waves to determine the stress in a load-bearing member. The '511 patent does not require an ultrasonic measurement to be taken when the load-bearing member is under zero stress. Rather, Holt provides a formula for stress calculation which is independent of the length of the fastener and, therefore, can be used to measure tensile stress in a fastener already under tension. The preferred embodiment uses phase detection for time-of-flight measurement. About 20-100 cycles of 5-10 MHz are transmitted, the transmitted and reflected signals are summed, and the frequency is adjusted for 180.degree. out-of-phase destructive interference or null. Holt also mentions previously disclosed time-of-flight measurement techniques as alternatives.
U.S. Pat. No. 3,918,294 (issued to Makino et al.) describes a method of measuring axial stresses in a bolt. An ultrasonic wave is applied to the bolt to generate forced oscillations and two different natural frequencies are measured in the bolt, one of which is measured when the bolt is under little or no axial force, the second of which is measured when the bolt is under axial stress. The ratio of change or the differential between the first and second frequencies is obtained and is compared to calibration data for the axial stress verses the ratio of change or differential.
Ultrasonic load measurement is a precise measurement technique for determining load in bolted joints. Pulse-echo techniques with removable ultrasonic transducers have been used in laboratories and for quality control for over thirty years. Historically, however, the practical difficulties in achieving reliable acoustic coupling and in incorporating transducers in tool drives have prevented this technique from becoming a general assembly tightening strategy. U.S. Pat. No. 4,846,001 (issued to Kibblewhite) teaches the use of a thin piezoelectric polymer film which is permanently, mechanically, and acoustically coupled to the upper surface of a member and is used to determine the length, tensile load, stress, or other tensile load-dependent characteristic of the member by ultrasonic techniques. Although the invention represented a significant advance over the prior state of the art in terms of performance, ease of manufacture, and manufacturing cost, there are disadvantages with a transducer of this construction. These disadvantages relate to environmental performance, in particular the maximum temperature limitations of the polymer material which restricts its application, and the possibility of the transducer, fixed to the fastener with adhesive, coming loose and causing an obstruction in or damage to a critical assembly.
These disadvantages were overcome by permanent transducer technology developed at Ultrafast, Inc. and disclosed, specifically, in U.S. Pat. No. 5,131,276 issued to Kibblewhite and assigned to Ultrafast, Inc. The '276 patent teaches a load-indicating member having an ultrasonic transducer, including an acoustoelectric film, grown directly on the fastener surface. By growing the acoustoelectric film directly on the fastener, the film is mechanically, electrically, and acoustically interconnected to the surface. This advance not only allows the precise pulse-echo load measurement technique to be used in production assembly but also significantly improves accuracies by eliminating errors that result from axial and radial movement of the removable transducer relative to the bolt and from variations in the coupling media.
With these errors eliminated, the greatest source of inaccuracy in ultrasonic load measurement is bending. Bending stresses in bolts can be caused by one or both of: (1) the "straightening" during tightening of a bolt that has a slight bend as a result of the forming or heat treatment manufacturing process, and (2) the tightening of a bolt in a joint in which the joint bearing surfaces are not parallel. When an ultrasonic wave or beam propagates through a fastener or other load-bearing member in the presence of stresses which are asymmetrical with respect to the axis of propagation, the beam is redirected due to both geometrical aberrations and the effects of material stress on sound velocity. In this situation, the amplitude, phase, and time-of-flight of the received ultrasonic echoes may vary and the measurements based upon them may be adversely affected. Thus, the prior art methods and devices are susceptible in terms of reliability and accuracy to bending stresses induced in a fastener during tightening and normal operation.
U.S. Pat. No. 5,029,480 (issued to Kibblewhite) discloses a load-indicating member (e.g., a fastener) with a shank having at least one external groove. An ultrasonic transducer (preferably a piezoelectric film transducer) is coupled to the load-indicating member so that an ultrasonic wave is directed to the groove. Thus, the groove acts as an artificial reflector by providing a face for reflecting the ultrasonic wave, generated by the transducer, back to the transducer. The head surface of the bolt may be fashioned to direct the acoustic signal toward the artificial reflector.
Although the prior art has discussed the use of radiused or focused surfaces to direct acoustic beams to specific reflecting surfaces (as in the case of artificial reflectors), no use of geometric end or head-shaping to correct for the effects of fastener bending on received echoes has been considered. U.S. Pat. No. 4,569,229 (issued to de Halleux) teaches a method for measuring stresses in load-bearing members which eliminates the need to calibrate for grip length. The method measures the time an echo travels from the top of a load-bearing member to an artificial reflector and back. The artificial reflector constitutes axial, radial, or both axial and radial bore holes or perforations in the load-bearing member. The transit time in the bolt depends on the stress in the bolt.
To overcome the shortcomings of prior fastener designs (particularly with respect to bending), a new fastener design is provided. Also provided is a method for determining that design. The central object of the present invention is to provide a method of making more accurate and reliable ultrasonic measurements in a load-bearing member by reducing the influence of geometric variations and asymmetrical stress in the member. A related object is to provide, through the use of the method of the present invention, fasteners that provide ultrasonic echoes which are significantly more robust in the presence of bending stresses.